1. Field of the Invention
The present invention relates to a track jumping circuit and a tracking servo circuit suitably used in an optical disk unit for optically recording and reproducing information. More particularly, the present invention relates to a jump pulse corrector and an optical disk unit in which a step jumping operation can be accurately performed and a track follow-up servo operation can be stabilized even when an actuator constant is changed in a track follow-up servo for displacing a light spot in a direction perpendicular to a track of an optical disk.
2. Description of the Related Art
In a general optical disk unit for optically recording and reproducing information, a light spot following up a track of an optical disk is moved onto an adjacent track at a high speed. Such a moving operation is called a track jumping operation or a step jumping operation. For example, the step jumping operation is described in detail in "Technique of Optical Disk", pp. 161-165 published by radio technique company on Feb. 10, 1989.
In this step jumping operation, the operating state of a track follow-up servo is set to an opening state. An accelerating pulse having a predetermined pulse height is transmitted to a tracking actuator. After a constant time, a decelerating pulse having a polarity inverse to that of the accelerating pulse is transmitted to the tracking actuator.
In this case, heights and time widths of the accelerating and decelerating pulses are designed such that the light spot is approximately moved by one track and a moving speed of the light spot is set to a desirable value such as zero when the pulses are completely applied.
However, when the optical disk unit is operated by using such constant pulse heights and time widths, the operation of the optical disk unit is influenced by dispersions in force constant of the actuator (generated thrust per unit electric current) and actuator mass. Further, in a system for applying a pulse voltage to the actuator, a moving acceleration of the actuator is dispersed by dispersion in resistance value of an actuator coil, etc.
Therefore, no moving amount of the light spot is equal to a length corresponding to one track and no moving speed of the light spot is equal to the desirable value when the movement of the light spot is completed.
Accordingly, in the general track follow-up servo, there is a problem that no light beam can be accurately positioned on an adjacent track.
Further, as mentioned above, when the force constant of the actuator, etc. are dispersed, a loop gain of the track follow-up servo is changed so that no track follow-up servo operation can be stably performed.